Multilayered printed wiring board

ABSTRACT

A multilayer printed wiring board comprises a plurality of insulating layers which is about 100 μm or less in thickness and a plurality of conductor circuits formed on the insulating layers. Each of a plurality of viaholes electrically connecting conductor circuits on the insulating layers to each other is formed tapered inwardly from the surface of the insulating layer and the viaholes are disposed opposite to each other to form a multistage stacked vias.

FIELD OF THE INVENTION

The present invention relates to a multilayer printed wiring board formounting electronic components such as a capacitor, IC and the like onthe surface layer thereof.

BACKGROUND ART

The recent portable electronic devices such as a mobile phone, digitalcamera, etc., use a multilayer printed wiring board having electroniccomponents such as a capacitor, IC and the like mounted on the surfacelayer thereof.

More specifically, the electronic components to be mounted on such awiring board are passive parts such as an IC chip, capacitor, resistor,inductor and the like, a liquid crystal device, a display device fordigital indication, control devices such as keypad, switches and thelike or external terminals such as a USB, earphone and the like.

Various conductor pads corresponding to electronic components areprovided together on a printed circuit board, a solder body is formed oneach of the conductor pads, and the electronic components are mounted onthe solder bodies on the conductor pads.

A typical one of multilayer circuit boards on which the above electroniccomponents are to be mounted is disclosed in the Japanese unexaminedpatent publication No. H10-13028. As disclosed in the above Japaneseunexamined patent publication No. H10-13028, holes for viaholes areformed by lasering, in an insulating rigid substrate having a conductorcircuit formed on one side or either side thereof, viaholes are formedby filling a metal paste or a plating material in the holes to form aninterlayer-connected circuit board, and two or more such circuit boardsprepared as above are stacked sequentially or collectively to produce amultilayer printed wiring board.

In such a multilayer circuit board, two circuit boards are electricallyconnected to each other by connecting a viahole or a land of the viaholein one of the adjacent circuit boards to a conductor circuit or land ofthe other circuit board.

Also, the circuit boards are joined, at other areas thereof notcontributing to electrical connection between them, to each other withan adhesive layer, prepreg or the like of a thermosetting resin, tothereby form a multilayer structure.

Normally, a solder resist layer to protect the conductor circuit isformed on the surface layer of the above multilayer circuit board orprevailing printed wiring board, a hole is formed in a part of thesolder resist layer, and a corrosion-resistant layer of gold or nickelis formed on the surface, exposed in the hole, of the conductor circuit.A solder body such as a solder bump is formed on the surface, having thecorrosion-resistant layer formed thereon, of the conductor circuit, andan electronic component such as a capacitor, IC or the like is mountedon the solder bodies formed on the conductor circuit surface.

BRIEF SUMMARY OF THE INVENTION

The inventors of the present invention have devoted themselves to thestudies in this field of technology for implementing the abovemultilayer printed wiring board. Taking into consideration the shape andstacked pattern of viaholes electrically connecting conductor circuitsto each other in the multilayer wiring board, the inventors took theviaholes formed in one of two outermost insulating layers and anotherinsulating layer disposed inside the one insulating layer as onesbelonging to a first group and those formed in the other of the twooutermost insulating layers and still another insulating layer disposedinside the other insulating layer as ones belonging to a second group,and tapered each of the viaholes included in each group and stacked insuch geometry that the groups were opposite to each other toward thesurface of an insulating substrate in which the viahole were formed ortoward the surface of the conductor circuit formed on the insulatingsubstrate. With this arrangement, the inventors found that the printedcircuit board would not be reduced in rigidity, warped or otherwiseinfluenced even if the insulating substrates included in the printedcircuit board was reduced in thickness. Based on these findings, theinventors worked out the present invention which will be describedbelow.

According to one embodiment of the present invention, there is provideda multilayer printed wiring board in which insulating layers andconductor layers are stacked alternately on each other and the conductorlayers are electrically connected to each other through viaholes formedin the insulation layers, wherein the viaholes include a first group ofviaholes which are formed in one of two outermost insulating layers andanother insulating layer disposed inside the one insulating layer and asecond group of viaholes which are formed in the other of the twooutermost insulating layers and still another insulating layer disposedinside the other insulating layer, each of the viaholes included in thefirst and second groups is formed tapered to have a diameter which issmaller in the direction of thickness of the insulating layer, and theinsulating layer is about 100 μm or less in thickness.

According to other embodiment of the present invention, there is alsoprovided a multilayer printed airing board in which one insulatingsubstrate having at least one conductor circuit formed thereon hasstacked on either side thereof at least one or more other insulatingsubstrates having at least one conductor circuit formed thereon and theconductor circuit formed on the one insulating substrate and that on theother insulating substrate are electrically connected to each otherthrough viaholes formed in the insulating substrates, wherein theviaholes formed in the other insulating substrate stacked on one side ofthe one insulating substrate form together a first group includingviaholes each formed tapered toward the surface of the insulatingsubstrate or that of the conductor circuit formed on the surface of theinsulating substrate, while the viaholes formed in the other insulatingsubstrate stacked on the other side of the one insulating substrate formtogether a second group including viaholes each formed tapered towardthe surface of the insulating substrate or that of the conductor circuitformed on the surface of the insulating substrate, and the insulatingsubstrate is about 100 μm or less in thickness.

According to still other embodiment of the present invention, there isalso provided a multilayer printed wiring board in which an innerinsulating substrate having at least one conductor circuit formedthereon has stacked on either side thereof at least one or more outerinsulating substrates having at least one conductor circuit formedthereon and the conductor circuit formed on the inner insulatingsubstrate and that on the outer insulating substrate are electricallyconnected to each other through viaholes formed in each of theinsulating substrates, wherein the viaholes formed in the outerinsulating substrate stacked on one side of the inner insulatingsubstrate form together a first group including viaholes formed taperedtoward the surface of the insulating substrate or that of the conductorcircuit formed on the surface of the insulating substrate while theviaholes formed in the outer insulating substrate stacked on the otherside of the inner insulating substrate form together a second groupincluding viaholes formed tapered toward the surface of the insulatingsubstrate or that of the conductor circuit formed on the surface of theinsulating substrate, and the inner or outer insulating substrate isabout 100 μm or less in thickness.

In the above multilayer printed wiring boards according to theembodiments of the present invention, the insulating layer or insulatingsubstrate may be about 50 μm or less in thickness.

Also in the above multilayer printed wiring boards according to theembodiments of the present invention, the first group of viaholes may bestacked on the second group of viaholes in such geometry that it isopposite to the second group of viaholes to form a multistage stackedvias structure. Otherwise, the first group of viaholes may be stacked onthe second group of viaholes in such geometry that it is shifted inrelation to the second group of viaholes in a direction generallyorthogonal to the direction of thickness of the insulating layer.

Also in the above multilayer printed wiring boards according to theembodiments of the present invention, the viaholes in the first orsecond group may be stacked on an almost straight line. Otherwise, theviaholes may be stacked together in such geometry that they are shiftedfrom each other in a direction generally orthogonal to the direction ofthickness of the insulating layer.

Also in the above multilayer printed wiring boards according to theembodiments of the present invention, the viaholes included in eitherthe first or second group may be positioned at two opposite apexes of animaginary tetragonal lattice on the insulating substrate while theviaholes included in the other group may be positioned at other twoopposite apexes of the imaginary tetragonal lattice on the insulatinglayer.

Also in the above multilayer printed wiring boards according to theembodiments of the present invention, the viaholes included in eitherthe first or second group may be positioned at apexes of an imaginarytetragonal or triangular lattice on the insulating substrate while theviaholes included in the other group may be positioned at the center ofthe imaginary tetragonal or triangular lattice on the insulating layer.

Also in the above multilayer printed wiring boards according to theembodiments of the present invention, the viaholes included in eitherthe first or second group may be positioned intensively in apredetermined area of the insulating substrate while the viaholesincluded in the other group may be positioned in a peripheral areasurrounding the predetermined area of the insulating layer.

Also in the above multilayer printed wiring boards according to theembodiments of the present invention, each of the viaholes may betapered at an inner angle of about 60 to about 90 degrees toward thesurface of the insulating substrate in which the viahole is formed orthe surface of the conductor circuit formed on the surface of theinsulating layer.

Also in the above multilayer printed wiring boards according to theembodiments of the present invention, each of the viaholes may be formedby filling a plating material in a hole formed in the insulatingsubstrate.

Also in the above multilayer printed wiring boards according to theembodiments of the present invention, the viaholes included in the firstand second groups may form a multistage stacked vias structure.

According to the other embodiment of the present invention, there isalso provided a multilayer printed wiring board in which insulatinglayers and conductor layers are stacked alternately on each other andthe conductor layers are electrically connected to each other throughviaholes formed in the insulating layers, wherein: the insulating layersare at least three in number and about 100 μm or less in thickness; theviaholes include a first group of viaholes and a second group ofviaholes; the first group includes viaholes in more than two stages ofstacked vias extending in the direction of thickness of the insulatinglayer and inwardly of the multilayer printed wiring board; and thesecond group includes viaholes formed tapered to have a diameter whichis smaller away from the first viahole group in the direction ofthickness of the insulating layer.

In the above multilayer printed wiring board according to the aboveembodiment of the present invention, the insulating layer may be about50 μm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of the multilayer printed wiringboard according to the present invention, showing the tapering of aviahole formed in the printed wiring board.

FIG. 2A is a schematic sectional view of a multistage stacked vias inthe multilayer printed wiring board according to the present invention,and FIG. 2B is a SEM photograph showing the section of a substratehaving the multistage stacked vias.

FIG. 3A is a schematic sectional view of a variant of the multistagestacked vias, FIG. 3B is a SEM photograph showing the section of thesubstrate, FIG. 3C is a schematic sectional view of another variant ofthe multistage stacked vias, and FIG. 3D is a SEM photograph showing thesection of the substrate.

FIG. 4 is a schematic sectional view of another basic shape of themultistage stacked vias in the multilayer printed wiring board accordingto the present invention.

FIGS. 5A to 5C schematically illustrate examples of disposition in planeof viaholes included in a multistage stacked vias.

FIG. 6 schematically illustrates another example of disposition in planeof viaholes included in a multistage stacked vias (disposition in theform of a triangular lattice).

FIG. 7 schematically illustrates another example of disposition in planeof viaholes included in a multistage stacked vias (disposition instraight lines).

FIGS. 8A and 8B schematically illustrate other examples of dispositionin plane of viaholes included in a multistage stacked vias (concentrateddisposition and dispersed disposition).

FIGS. 9A to 9E are schematic sectional views of a multilayer printedwiring board as an embodiment 1 of the present invention, showing somesteps in the manufacturing process for the multilayer printed wiringboard.

FIGS. 10A to 10E are schematic sectional views of the multilayer printedwiring board as the embodiment 1 of the present invention, showing someother steps in the manufacturing process for the multilayer printedwiring board.

FIG. 11 is a schematic sectional view of the multilayer printed wiringboard as the embodiment 1 of the present invention, showing another stepin the manufacturing process for the multilayer printed wiring board.

FIGS. 12A and 12B are schematic sectional views of the multilayerprinted wiring board as the embodiment 1 of the present invention,showing further steps in the manufacturing process for the multilayerprinted wiring board.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The multilayer printed wiring board as one embodiment of the presentinvention is such that viaholes to electrically connect conductor layersstacked together via an insulating layer include a first group ofviaholes formed in at least one insulating layer stacked inside one oftwo outermost insulating layers and a second group of viaholes formed ina least one insulating layer stacked inside of the other of the twooutermost insulating layers, the viaholes included in the first andsecond groups are formed tapered to have a diameter which is smaller inthe direction of thickness of the insulating layer, and the insulatinglayer is about 100 μm or less in thickness.

Namely, in the multilayer printed wiring board as a lamination formedfrom the conductor layers and insulating layers stacked alternately onone another, a multistage stacked vias are formed in which the viaholesformed in at least one insulating layer stacked inside one of the twooutermost insulating layers (first viahole group) and those formed in atleast one insulating layer stacked inside the other of the two outermostinsulating layers (second viahole group) are positioned opposite to eachother, the viaholes included in the multistage stacked vias are taperedtoward the surface of the insulating layer or that of the conductorlayer formed on the surface of the insulating layer, and each of theinsulating substrates is about 100 μm or less in thickness.

In the above multilayer printed wiring board as the embodiment of thepresent invention, the multistage stacked vias are formed in which thefirst group of viaholes formed in an insulating layers as the core ofstacking and an insulating layer stacked on one side of the formerinsulating layer and the second group of viaholes formed in aninsulating layer stacked on the other side of the insulating layer asthe core of stacking, the viaholes in each of the groups are tapered tohave a diameter which is smaller in the direction of thickness of theinsulating layer, that is, tapered toward the surface of the insulatinglayer or that of the conductor circuit formed on the insulating layer.Thus, even if the stacked insulating layer or substrate is about 100 μmor less in thickness, it can be prevented from being warped due to anexternal stress (such as an impact caused by dropping or the like). As aresult, the printed circuit board can have a satisfactory reliabilityand drop-shock resistance.

Also, the multistage stacked vias formed in the insulating layers willfunction as a stake to make it difficult for the insulating layers orinsulating substrates to be warped. Therefore, the substrates will beable to keep their flatness, so that the multilayer printed wiring boardcan maintain the reliability provable by the reliability test such as aheat cycle test.

Especially, a printed circuit board can advantageously be formed byforming a conductor circuit on an insulating layer or substrate of about100 μm or less in thickness and stacking more than one such aninsulating layer or substrate together.

Further, a printed circuit board can advantageously be formed by forminga conductor circuit on an insulating layer or substrate of about 50 μmor less in thickness and stacking more than one such an insulating layeror substrate together. It can be inferred that the printed circuit boardwill incur less reduction of its reliability and drop-shock resistance.

Also, the multistage stacked vias formed in such geometry that the firstand second viahole groups are opposite to each other effectivelyprevents the insulating layers or substrates from being warped bothoutwardly and inwardly of them. As a result, the printed circuit boardcan have a satisfactory reliability and drop-shock resistance.

Also, the multistage stacked vias disposed opposite to each other assurea satisfactory rigidity of the insulating substrates themselves.

Various embodiments of the multilayer printed wiring board according tothe present invention will be described in detail below with referenceto the accompanying drawings:

The insulating layer or substrate used in the multilayer printed wiringboard according to the present invention should be a rigid substrateselected from among, for example, glass cloth-based epoxy resin, phenolresin, glass cloth-based bismaleimide triazine resin, glass cloth-basedpolyphenylene ether resin, aramid unwoven cloth-based epoxy resin,aramid unwoven cloth-based polyimide resin, etc. The substrate of suchan insulating resin should desirably be about 100 μm or less inthickness. Also, the substrate of such an insulating resin may be about50 μm or less in thickness.

A multilayer printed wiring board (printed circuit board for mountingelectrical components) is formed by stacking such insulating layers orsubstrates alternately on the surface of a circuit board as the core ofstacking, in which at least one conductor circuit is formed on one orboth of the surfaces of the insulating layer or substrate. Also, thethickness of the multilayer printed circuit board itself can be reducedby reducing the thickness of all the insulating layers or substrates insuch a printed circuit board to about 100 μm or less.

Also, in the embodiment of the present invention, both the conductorcircuits formed on the insulating substrates and the viaholes(multistage stacked vias) included the first and second groups shoulddesirably be formed by plating.

The viahole is formed by electrolytic or electroless plating. The metalused for plating may be an element such as copper, nickel, iron, cobaltor the like or an alloy based on any of the elements.

In the embodiment of the present invention, the multistage stacked viasinclude viaholes formed tapered to have a diameter which is smaller inthe direction of thickness of the insulating layers having the viaholesformed therein, tapered toward the surface of the insulating substrateor that of the conductor layer formed on the surface of the insulatingsubstrate, or tapered from outside toward inside of the stackedinsulating layers, as shown in FIG. 1.

Typically, the multistage stacked vias should desirably be formedgenerally as a truncated cone whose top is larger in area than thebottom and section (generally trapezoidal) tapered in the direction ofthickness of the insulating substrate has an inner angle of about 60 toabout 90 degrees.

It is inferred that with the inner angle being within the above range,the insulating substrate can be limited without cancellation of theanchoring effect of the multistage stacked vias.

The viahole diameter at the bottom of the multistage stacked vias (willbe referred to as “via-bottom diameter” hereunder) may be at least about10 μm. The reason for the above is that since the viahole is formed byplating, the via-bottom diameter should be at least about 10 μm for thatplating. Thus, the upper conductor layer (upper conductor circuit andviahole) and lower conductor layer can electrically be connected to eachother

In the embodiment of the present invention, the multistage stacked viasshould preferably be formed so that the bottoms of outer viaholes (upperviaholes) are superposed on those of inner viaholes (lower viaholes) inthe same position. That is, the viaholes in the first and second groupscan be formed on almost same straight lines, respectively, as shown inFIGS. 2A and 2B.

Also, as long as the bottom of the upper viahole and that of the lowerviahole overlap each other, the multistage stacked vias can perform thefunction assured by its tapered shape. Further, in the first or secondgroup, the viaholes may be stacked together in respective positionsshifted in a direction in which they are per-endicular to each other inthe direction of thickness of the insulating layers and where thebottoms of the viaholes overlap at least partially each other.

For example, the viaholes in the first or second group may be stacked inrespective positions where they are shifted about a half of theirdiameter from each other as shown in FIGS. 3A and 3B. Also, the viaholesin the first or second group may be stacked in respective positionswhere they are shifted generally one viahole diameter from each other,as shown in FIGS. 3C and 3D.

Also in case the multilayer printed wiring board is used as usual, themultistage stacked vias can perform the function assured by its taperedshape.

Also, in the embodiment of the present invention, the first or secondgroup of viaholes included in the multistage stacked vias shouldpreferably be formed by providing at least two or more insulatingsubstrates and stacking the viaholes formed in the insulating substratestogether. That is, the first or second viahole group may be formed bystacking three, four or more viaholes together.

In the multistage stacked vias, the first and second viaholes groups maybe equal in number of viahole layers to each other (e.g., three viaholelayers in the first group and three viahole layers in the second group)or may be different in number of viahole layers from each other (e.g.,two viahole layers in the first group and three viahole layers in thesecond group). Basically, the interlayer electrical connection andreliability of the printed circuit board can be assured by forming thefirst and second viahole groups in the multistage stacked vias in suchgeometry that they are opposite to each other.

In the embodiment of the present invention, the multistage stacked viasmay be an electrically conductive layer. But it may also be anelectrically nonconductive layer, that is, a dummy conductor layer. Incase the multistage stacked vias are formed from a dummy conductorlayer, it will be assured that conductor layers other than the dummylayer (conductor layers existing around the dummy conductor layer orconductor layers electrically connected to each other by the viahole inthe multistage stacked vias or the like) will provide the same effect asthe electrically conductive layer does.

Also in the embodiment of the present invention, the first and secondviahole groups included in the multistage stacked vias should desirablybe disposed in the generally same positions (on the nearly same lines)within an area where the conductor circuit of each insulating substrateis formed as shown in FIGS. 2A and 2B or disposed in respectivepositions shifted from each other (dispersed disposition) as shown inFIGS. 3A to 3D.

By dispersing the first and second viahole groups uniformly in theentire area of the insulating substrates, for example, it is possible toassure a satisfactory resistance of the insulating substrates against awarp caused by an external stress.

Also, by concentrically disposing the first and second viahole groupsmainly in the central portion of the insulating substrate which islikely to be most influenced by an external stress, it is possible toassure a satisfactory resistance of the insulating substrates against awarp of the insulating substrates caused by an external stress.

Also, the first and second viahole groups may be disposed not in thecentral portion of the insulating substrate but mainly in a peripheralarea surrounding that central portion. Such a disposition of the firstand second viahole groups permits to assure the satisfactory resistanceof the insulating substrates against the warp. Also it is possible toassure a satisfactory flatness of the printed circuit board.

Further, mainly in the central portion of the insulating substrate, thefirst and second viahole groups may be disposed opposite to each other,while in the peripheral portion, the first and second viahole groups maybe disposed in respective positions shifted from each other.

The multistage stacked vias are disposed in any of various patterns ofdisposition in plane, such as a tetragonal lattice (as in FIGS. 5A to5C), triangular lattice (as in FIG. 6) or straight line (as in FIG. 7)in addition to the aforementioned patterns.

In the disposition in the tetragonal lattice pattern, the viaholes aredisposed as follows. For example, the first and second viahole groupsare disposed regularly in an imaginary tetragonal matrix as shown inFIG. 5A. The first viahole groups are disposed in an imaginary matrixwhile the second viahole groups opposite to the first ones are disposedin an intermediate portion of the matrix, as shown in FIG. 5B.Otherwise, the first and second viahole groups are disposed regularly inan imaginary zigzag matrix as shown in FIG. 5C.

In the disposition in the triangular lattice pattern, for example, thefirst viahole groups are disposed in an imaginary triangular patternwhile the second viahole groups opposite to the first ones are disposednear the center, or at the center of gravity, of the triangle, as shownin FIG. 6.

Also, the viaholes may be disposed on the straight lines. For example,at least two first groups of viaholes are disposed on an imaginarystraight line while a second viahole group opposite to the first viaholegroups is disposed at the center of the straight line, as shown in FIG.7.

Also, the multistage stacked vias may be formed in a combination of twoor more of the above-mentioned patterns of viahole disposition.

Further, in the embodiment of the present invention, for example, thesecond viahole groups may be disposed opposite to each other in areaswhere no first viahole groups are disposed, which disposition patternmay be added to the above-mentioned ones. Namely, the first viaholegroups may be disposed in a plane of matrix while the second viaholegroups may be disposed in a matrix in areas where no first viaholegroups are disposed. Alternatively, the first viahole groups may bedisposed mainly in the central portion of a substrate while the secondviahole groups may be disposed around the substrate (as in FIG. 8A).

Note that in FIGS. 5 to 8, the first viahole groups are indicated with asmall circle (∘) and the second viahole groups are indicated each with acrisscross (×) but the viahole groups (first groups, for example) may bedisplaced in other groups' (second groups, for example) positions. Thefirst and second viahole groups may be equal or different in diameter toor from each other.

There will be described in detail below one example of the method ofproducing a multilayer printed wiring board as an embodiment of thepresent invention.

(1) A starting material for producing the multilayer printed wiringboard as the embodiment of the present invention may be a circuit boardused as a basic unit for the multilayer printed wiring board and havingattached a copper foil on one or either side of an insulating substratethereof.

The insulating substrate is a rigid one selected from among, forexample, glass cloth-based epoxy resin, glass cloth-based bismaleimidetriazine resin, glass cloth-based polyphenylene ether resin, aramidunwoven cloth-based epoxy resin, aramid unwoven cloth-based polyimideresin, etc. The most desirable one is the glass cloth-based epoxy resinsubstrate.

The insulating substrate should desirably be about 100 μm or less inthickness. Also, the insulating substrate may be about 50 μm or less inthickness.

According to the present invention, a hole for forming a viahole may beformed by lasering in the circuit board by either a direct laseringmethod in which lasering is made to form a hole through a copper foiland insulating substrate at a time or a conformal method in which acopper foil is etched to remove a portion thereof corresponding to aviahole and then a hole is formed by lasering in the insulatingsubstrate.

The copper foil attached over the insulating substrate should desirablybe about 5 to about 20 μm in thickness.

The above range of the copper foil thickness assures that the end faceof the copper foil in the position of a viahole will not be deformedwhen a hole for the viahole is formed in the insulating substrate, aconductor circuit having a predetermined shape can easily be formed anda circuit board pattern having fine line width can easily be formed byetching.

The copper foil may be adjusted in thickness by a half etching. In thiscase, the copper foil thickness may be larger than the above and thethickness of the etched copper foil is adjusted to within the aboverange.

Also, in case a double-side copper-clad laminate is used as the circuitboard, the copper thickness may be within the above range but the copperfoil may be different in thickness from one side to the other of thecircuit board as long as the thickness is within that range, whereby itis possible to proceed with subsequent steps of the manufacturingprocess smoothly.

The circuit board should preferably be formed from a single- ordouble-sided copper-clad laminate produced by hot-pressing of a stack ofa copper foil and a B-stage prepreg as an insulating substrate formed byimpregnating epoxy resin into a glass cloth.

The reason for the above is that the wiring pattern and viaholes canaccurately be positioned without displacement thereof in subsequentsteps of the manufacturing process after the copper foil is etched.

(2) Next, holes for viaholes are formed by lasering in the insulatingsubstrate.

In case a single-sided copper-clad laminate is used to form the circuitboard, the surface of the insulating substrate opposite to the sidewhere the copper foil is attached is irradiated with carbon dioxidelaser to form a hole through the insulating substrate to the copper foil(or conductor circuit pattern).

In case a double-sided copper-clad laminate is used to form the circuitboard, the surface of the insulating substrate opposite to the sidewhere the copper foil is attached is irradiated with carbon dioxidelaser to form a hole through both the copper foil and insulatingsubstrate to the copper foil (or conductor circuit pattern) attached onthe other side of the insulating substrate, or a hole smaller indiameter than the viahole is formed by etching in the one side of thecopper foil attached on the insulating layer and then carbon dioxidelaser is irradiated to the smaller hole to form a hole through theinsulating substrate to the copper foil (or the conductor circuitpattern) attached on the other side of the insulating substrate.

The lasering is done with the use of a pulse oscillation type carbondioxide lasering unit for the side wall of the viahole hole to betapered at an angle of about 60 to about 90 degrees toward the surfaceof the insulating substrate.

The lasering may be made with, for example, a pulse energy of about 0.5to about 100 mJ, pulse width of about 1 to about 100 μs, pulse intervalof about 0.5 ms or more and 2 to 10 shots, and with a laser under theseconditions within this range, the taper angle of the side walls of thehole can be adjusted.

The diameter of the viahole hole formed under the above conditionsshould desirably be about 50 to about 250 μm. With a lasering under theconditions within the above-specified range, the side walls canpositively be tapered and wiring density can be increased.

(3) The above laminate may be subjected to desmearing in order to removeresin residues from on the side walls and bottom of the hole formed instep (2) above.

The desmearing may be made by wet processing with acid or oxidizingagent (e.g., chromic acid or permanganic acid) or dry processing such asoxygen plasma discharge, corona discharge, ultraviolet lasering, excimerlasering or the like.

Any one of the above desmearing methods is selected taking inconsideration of a residual-resin amount expected based on the type andthickness of an insulating substrate used, diameter of a viahole formedin the circuit board, lasering conditions, etc.

(4) Next, the surface of the copper foil on the desmeared insulatingsubstrate is electrolytic plated with the copper foil being used as aplating lead to fill electrolytic copper into the hole to the fullextent, to thereby form a viahole (filled via).

Note that after the electrolytic-copper plating, the electrolytic copperplate protuberant on the upper portion of the viahole in the insulatingsubstrate may be removed and flattened by belt sanding, buffing, etchingor the like as the case may be.

Also, after the copper foil is electroless-plated, it may be subjectedto electrolytic plating. In this case, the copper foil may beelectroless-plated with copper, nickel or silver or the like.

(5) Next, an etching resist layer is formed over the electrolytic-coppercoating on the insulating substrate in the step (4) above. The etchingresist layer may be formed either by applying a resist liquid or byattaching a pre-formed film of the resist liquid. A mask having acircuit pattern pre-depicted thereon is mounted on the resist layer, anetching resist layer is formed by exposing and developing the resistlayer, and the metal layer in an area other than the area where theetching resist layer is not formed is etched to form a conductor circuitpattern including a conductor circuit and land.

The etchant used in the above processing should desirably be at leastone selected from among aqueous solutions of sulfuric acid-hydrogenperoxide, persulfate, cupric chloride and ferric chloride.

To easily form a fine conductor circuit pattern prior to forming aconductor circuit by etching the copper foil and electrolytic-coppercoating, the electrolytic-copper coating may be pre-adjusted inthickness by etching the entire surface of the coating.

The land as a part of the conductor circuit should desirably be formedto have an inside diameter generally equal to the viahole diameter or tohave an outside diameter larger than the viahole diameter, so that theland diameter will be within a range of about 75 to about 350 μm. Thereason for the above is that with the land diameter being within theabove range, the multistage stacked vias can perform its function evenif the viaholes are displaced in relation to each other.

An insulating resin layer and copper foil are stacked on one side oreither side of the circuit board formed through the above steps (1) to(5) and which functions as the core of stacking. Thus, the circuit boardwill include one or two more insulating resin layers.

A viahole and conductor circuit are formed on the stacked insulatingresin layers through similar steps to the above steps (2) to (5),and-with an insulating resin layer and copper foil being stacked on theconductor layer, steps similar to the above steps (2) to (5) can berepeated to form a printed wiring board including further layers.

In the aforementioned method, the insulating resin layers are stackedsequentially to form a multilayer insulating resin layer structure.However, two or more circuit boards each including one insulating resinlayer may collectively be stacked and hot-pressed to form a multilayerprinted wiring board.

In the multilayer printed wiring board formed as above, the viaholeformed in each of the stacked circuit boards or insulating resin layersis tapered to have an inner angle of about 60 to about 90 degrees. inrelation to the surface of the circuit board or insulating resin layerin which the viahole is formed. The viaholes formed in at least one ormore insulating resin layers including a circuit board as the core ofstacking form together a first group while those formed in at least oneor more other insulating resin layers stacked together oppositely to theinsulating resin layers in which the viaholes in the first group areformed form together a second group. These first and second viaholegroups form together a multistage stacked vias. Each viahole in thefirst and second viahole groups should desirably be tapered at an innerangle of 60 to 90 degrees. toward the surface of the insulating resinlayer in which the viahole is formed.

(6) Next, a solder resist layer is formed over the surface of each ofthe outermost circuit boards. In this case, a solder resist compositionis coated over the outer surface of the circuit board, the coating isdried, a photo mask having depicted thereon a hole for a solder pad ismounted on the dried coating, and the solder resist is exposed anddeveloped to form a solder pad hole in which there is exposed aconductive pad portion positioned just above the viahole in theconductor circuit. In this case, a dry film of a solder resist isattached over the solder resist coating, exposed and developed orirradiated with laser light to form the solder-pad hole.

A corrosion-resistant layer of nickel, gold or the like is formed on thesolder pad exposed in an area where the photomask is not formed. When itis formed from nickel-gold, the nickel layer should desirably be about 1to about 7 μm in thickness while the gold layer should desirably beabout 0.01 to about 0.1 μm in thickness. The corrosion-resistant layermay be formed from a nickel-palladium-gold alloy, gold, copper or thelike in addition to the nickel-gold.

After the corrosion-resistant layer is formed, the mask layer is peeledoff. Thus, a printed wiring board is provided which includes the solderpad having the corrosion-resistant layer formed thereon and the solderpad having no such corrosion-resistant layer formed thereon.

(7) Solder is supplied from the hole in the solder resist layer formedin the above step (6) to the solder pad exposed just above the viahole,melted and solidified to form a solder bump, or a conductive ball orconductive pin is joined to the solder pad with the use of a conductiveadhesive or solder layer to form a multilayer circuit board.

The solder and solder layer may be supplied by solder transfer orprinting.

The solder transfer is such that a solder foil is attached to over aprepreg and etched with leaving only a portion corresponding to a holeto form a solder pattern and thus a solder carrier film, a flux isapplied to the hole in the solder resist on the circuit board, then thesolder carrier film is stacked for the solder pattern to touch thesolder pad, and it is heated for transfer of the solder.

On the other hand, the solder printing is such that a printing mask(metallic one) having a hole formed therein in a position correspondingto a solder pad is placed on the circuit board and then a solder past isprinted and heated. The solder use to form such a solder bump may be anSn—Ag solder, Sn/In solder, Sn/Zn solder or Sn/Bi solder.

EXAMPLE 1

(1) First, there is produced a circuit board as one unit included in amultilayer printed wiring board. The circuit board is the core ofstacking a plurality of insulating layers which are to be stackedtogether. For producing such a circuit board, copper foils and a B-stageprepreg 12 prepared by impregnating epoxy resin into a glass cloth arestacked together and hot-pressed to form a double-sided copper-cladlaminate 10 which is to be used as a starting material (as in FIG. 9A).

The prepreg or insulating substrate 12 is 60 μm in thickness and copperfoils 14 are 12 μm in thickness. The copper foil 14 may be more than 12μm in thickness. In this case, the copper foil 14 is adjusted by etchingto have a thickness of 12 μm.

(2) The double-sided copper-clad laminate 10 including the copper foils14 is irradiated with carbon dioxide laser to form a viahole-forminghole 16 through one of the copper foils 14 and insulating substrate 12to the copper foil 14 on the other side of the insulating substrate 12,and then the hole formed by the lasering is desmeared by chemicalprocessing with permanganic acid (as in FIG. 9B).

Note that in this Example, the viahole-forming hole 16 is formed using ahigh-peak short-pulse oscillation type carbon dioxide lasering unit (bythe Hitachi Via). A copper foil on a glass cloth-based epoxy resinsubstrate of 60 μm in thickness having a 12 μm-thick copper foilattached thereon is irradiated directly with a laser beam under thefollowing conditions to form a hole 16 of 75 μm in diameter at a speedof 100 holes/sec.

(Lasering Conditions) Pulse energy: 0.5 to 100 mJ Pulse width: 1 to 100μs Pulse interval: 0.5 ms or more Number of shots: 2 Oscillatingfrequency: 2000 to 3000 Hz

The hole 16 thus formed under the above conditions has an inner wallhaving a generally truncated-conical shape and a taper angle (innerangle) of 65 degrees in relation to the surface of the insulatingsubstrate 12.

(3) The surface of the copper foil 14 having the desmearedviahole-forming hole 16 formed therein is electrolytic plated with thecopper foil 14 being taken as a plating lead and under the followingelectrolytic plating conditions to form an electrolytic copper platedfilm (as in FIG. 9C):

(Electrolytic Plating Solution) Sulfuric acid: 2.24 mol/l Coppersulfate: 0.26 mol/l Additive A (reaction promoter): 10.0 ml/l Additive B(reaction inhibitor): 10.0 ml/l (Electrolytic plating conditions)Current density: 1 A/dm² Time: 65 min Temperature: 22 ± 2° C.

The electrolytic copper plating in the viahole-forming hole is promotedby the additive A while the electrolytic copper plated film mainly onthe copper foil is inhibited by the additive B from being formed. Also,when the electrolytic copper is fully filled in the viahole-forming holeup to nearly the same level as the surface of the copper foil 14, theadditive B will adhere to the copper foil 14 to inhibit the electrolyticcopper plated film from being formed as on the copper foil.

Thus, there is formed a viahole 20 having the electrolytic copper filledin the hole 16 so that the surface of the viahole 20 will be nearlyflush with the copper foil surface.

Also, the conductor layer of the copper foil 14 and electrolytic copperplated film may be adjusted in thickness by etching. The thickness ofthe conductor layer may be adjusted in thickness by a physical methodsuch as the sand-belt polishing or buffing.

(4) At either side of the circuit board formed as in the step (3) above,a dry film of a photoresist is formed to a thickness of 15 to 20 μm onthe conductor layer of the copper foil 14 and electrolytic copper platedfilm. A mask having depicted thereon a conductor circuit including aviahole land is mounted on the resist film, and the resist film isexposed and developed to form an etching resist layer 22 (as in FIG.9D). The copper foil 14 and electrolytic copper plated film exposed in aportion where the etching resist is not formed are dissolved and removedby etching with an etching solution of hydrogen peroxide liquid/sulfuricacid.

(5) Thereafter, the etching resist layer 22 is peeled using an alkalinesolution to form a pattern 24 of a conductor circuit including a viaholeland. Thus, there is formed a viahole 20 electrically connecting theconductor circuits on both the front and rear sides of the circuit boardto each other to provide a circuit board in which the viahole 20 and acopper foil portion forming the conductor circuit 24 are flat (as inFIG. 9E).

(6) At the front and rear sides of the circuit board formed through theabove steps (1) to (5), there are superposed a 60 μm-thick B-stageprepreg prepared by impregnating epoxy resin into a glass cloth and 12μm-thick copper foils. The prepreg and copper foils are hot-pressed at atemperature of 80 to 250° C. and under a pressure of 1.0 to 5.0 kgf/cm²to stack 60 μm-thick insulating resin layers 26 and 12 μm-thickconductor layers 28 on the circuit board (as in FIG. 10A).

(7) Next, nearly as in the step (2) above, the circuit board isirradiated at either side thereof with carbon dioxide laser under thefollowing conditions to form a 65 μm-diameter viahole-forming hole 30through the insulating resin layer 26 and conductor layer 28 to thelower conductor circuit 24 at a speed of 100 holes/sec. Then, the holeformed by the lasering is desmeared by chemical processing withpermanganic acid (as in FIG. 10B).

(Lasering Conditions) Pulse energy: 0.5 to 100 mJ Pulse width: 1 to 100μs Pulse interval: 0.5 ms or more Number of shots: 2 Oscillatingfrequency: 2000 to 3000 Hz

The hole 30 thus formed under the above conditions has an inner wallhaving a generally truncated-conical shape and a taper angle (innerangle) of 65 degrees in relation to the surface of the insulating resinlayer 26.

(8) Nearly as in the step (3) above, the conductor layer 28 at the sideof the desmeared viahole-forming hole 28 is electrolytic plated underthe following electrolytic plating conditions to form an electrolyticcopper plated film 32 (as in FIG. 10C):

(Electrolytic Plating Solution) Sulfuric acid: 2.24 mol/l Coppersulfate: 0.26 mol/l Additive A (reaction promoter): 10.0 ml/l Additive B(reaction inhibitor): 10.0 ml/l (Electrolytic plating conditions)Current density: 1 A/dm² Time: 65 min Temperature: 22 ± 2° C.

Thus, a viahole 34 having an electrolytic copper 32 filled in the hole30 is formed so that the surface of the viahole 34 will generally beflush with the copper foil surface.

(9) Nearly as in the step (4) above, a dry film of a photoresist isformed to a thickness of 15 to 20 μm on the electrolytic copper platedlayer formed in the above step (8). A mask having depicted thereon aconductor circuit, land of the viahole 34, etc. is mounted on the resistfilm, a second positioning mark is imaged by a camera, and the resistfilm is exposed and developed to form an etching resist layer 36 (as inFIG. 10D).

Thereafter, the electrolytic copper plated film and copper foilcorresponding to a portion where the etching resist is not formed areremoved by etching with an etching solution of hydrogen peroxideliquid/sulfuric acid.

(10) Next, the etching resist layer 36 is peeled using an alkalinesolution to form a conductor circuit 38 including the viahole 34 andland of the latter. Thus, there is formed a circuit board in which theviahole 34 electrically connecting both the front and rear sides of thecircuit board to each other and a copper foil portion forming theconductor circuit 38 are flat (as in FIG. 10E).

Further, the above steps (6) to (10) are repeated to form one moreinsulating resin layer 40, an electrolytic copper is filled by platinginto a hole formed in the insulating resin layer 40 to form a viahole 42and also a conductor circuit pattern 44 including a viahole land. Thus,there can be provided a multilayer printed wiring board in which twoinsulating layers and conductor circuits are formed on either side ofthe double-sided circuit board 10 (as in FIG. 11).

More specifically, the multilayer printed wiring board includes fiveinsulating layers and six conductor circuits. In this multilayer printedwiring board, the viaholes formed in the double-side circuit board andtwo insulating layers stacked on the upper side of the double-sidedcircuit board form together a first group of viaholes each having atruncated-conical section tapered at an angle of 65 degrees. toward thesurface of the insulating layer, and the viaholes formed in twoinsulating layers stacked on the lower side of the double-sided circuitboard form together a second group of viaholes each having atruncated-conical section taped at an angle of 65 degrees. toward thesurface of the insulating layer. These first and second viahole groupsare disposed opposite to each other and stacked on almost same straightlines, respectively.

(11) A solder resist layer 46 is formed on the surface of each of twoinsulating layers on the outermost sides, respectively, of the circuitboard formed in the step (10) above to form a solder resist layer 46.

First, a film-shaped solder resist of 20 to 30 μm in thickness isattached on the insulating layer having the conductor circuit 38 formedthereon. Next, the insulating layer is dried at 70° C. for 20 min andthen at 100° C. for 30 min, and then a 5 mm-thick soda lime glasssubstrate having depicted thereon, with a chrome layer, a circle pattern(mask pattern) for a hole in the solder resist is attached at the sidethereof having the chrome layer closely to the solder resist layer 46,exposed to ultraviolet rays of 1000 mJ/cm² and DMTG-developed.

Further, the above assembly is heated at 120° C. for 1 hour and then at150° C. for 3 hours to form a 20 μm-thick solder resist layer 46 havinga hole 48 (200 μm in diameter) corresponding to a pad (as in FIG. 12A).

Before the solder resist layer 46 is formed over the surface of theoutermost insulating layer of the multilayer printed wiring board, aroughed-surface layer may be provided as necessary.

(12) Next, the circuit board having the solder resist layer 46 formedthereon is immersed in an electroless nickel plating solution containing30 g/l of nickel chloride, 10 g/l of nickel chloride, 10 g/l of sodiumhypophosphite and 10 g/l of sodium citrate and having a pH value of 5for 20 min to form a 5 mμ-thick nickel plated layer on the surface ofthe conductor circuit 38 exposed from the hole 48.

Further, the circuit board is immersed in an electroless nickel platingsolution containing 2 g/l of potassium gold cyanide, 75 g/l of ammoniumchloride, 50 g/l of sodium citrate and 10 g/l of sodium hypophosphite at93° C. for 23 sec to form a 0.03 μm-thick gold plated layer on thenickel plated layer, to thereby form a conductor pad 50 covered with ametal layer consisting of the nickel and gold plated layers.

(13) Then, with a metal mask being mounted on the solder resist layer46, a solder paste of Sn/Pb or Sn/Ag/Cu having a melting point T2 of183° C. is printed. The metal mask is removed from on the solder resistlayer 46, and then the solder paste is subjected to reflowing at atemperature of about 183° C. to provide a multilayer printed wiringboard having a solder layer 52 formed on the conductor pad 50 exposed inthe hole 48 (as in FIG. 12B).

Next, mainly electronic components such as a capacitor, resistor and thelike are mounted in an area where the solder layer 52 is formed, andexternal terminals such as a key pad and the like are mounted in an areawhere the solder layer 52 is not formed. Thus, a multilayer printedwiring board is provided.

EXAMPLE 2

A multilayer printed wiring board is produced generally similarly to theExample 1 except that each of the viaholes included in the first andsecond viahole groups formed in the insulating layers, respectively,stacked on the front and rear sides, respectively, of the double-sidedcopper-clad laminate are shifted about a half of the viahole diameterfrom each other as shown in FIG. 3A.

EXAMPLE 3

A multilayer printed wiring board is produced generally similarly to theExample 1 except that each of the viaholes included in the first viaholegroup formed in the insulating layer stacked on the upper side of thedouble-side copper-clad laminate and those included in the secondviahole group formed in the insulating layer stacked on the lower sideof the double-sided copper-clad laminate are shifted about a half of theviahole diameter from each other, as shown in FIG. 3B.

EXAMPLE 4

A multilayer printed wiring board is produced generally similarly to theExample 1 except that two insulating layers are stacked on the upperside of the double-sided copper-clad laminate while one insulating layeris stacked on the lower side of the double-sided copper-clad laminate sothat four insulating layers and five conductor circuits are formed.

EXAMPLE 5

A multilayer printed wiring board is produced generally similarly to theExample 2 except that two insulating layers are stacked on the upperside of the double-sided copper-clad laminate while one insulating layeris stacked on the lower side of the double-sided copper-clad laminate sothat four insulating layers and five conductor circuits are formed.

EXAMPLE 6

A multilayer printed wiring board is produced generally similarly to theExample 3 except that two insulating layers are stacked on the upperside of the double-sided copper-clad laminate while one insulating layeris stacked on the lower side of the double-sided copper-clad laminate sothat four insulating layers and five conductor circuits are formed.

EXAMPLE 7

A multilayer printed wiring board is produced generally similarly to theExample 1 except that the first viahole group formed in the insulatinglayers stacked on the double-sided copper-clad laminate and on the upperside of the latter is horizontally shifted generally a half of theviahole diameter from the second viahole group formed in the insulatinglayers stacked on the lower side of the double-sided copper-cladlaminate, as shown in FIG. 4.

EXAMPLE 8

A multilayer printed wiring board is produced generally similarly to theExample 7 except that two insulating layers are stacked on the upperside of the double-sided copper-clad laminate while one insulating layeris stacked on the lower side of the double-sided copper-clad laminate sothat four insulating layers and five conductor circuits are formed.

EXAMPLE 9

A multilayer printed wiring board is produced generally similarly to theExample 4 except that the viaholes are stacked together for thoseincluded in the first group to be positioned at the respective apexes ofan imaginary tetragonal lattice (lattice interval: 10 mm) on theinsulating substrate and those included in the second group to bepositioned at the center of the imaginary tetragonal lattice, as shownin FIG. 5B.

EXAMPLE 10

A multilayer printed wiring board is produced generally similarly to theExample 4 except that the viaholes are stacked together for thoseincluded in the first group to be positioned at the respective apexes ofan imaginary triangular lattice (lattice interval: 20 mm) on theinsulating substrate and those included in the second group to bepositioned at the center of the imaginary triangular lattice, as shownin FIG. 6.

EXAMPLE 11

A multilayer printed wiring board is produced generally similarly to theExample 4 except that the viaholes are stacked together for thoseincluded in the first group to be positioned nearly at the center of theinsulating substrate and concentrically disposed in an area of 40×40 mmand those included in the second group to be positioned outside thecentral area of a peripheral area (40×40 mm) surrounding the centralportion and inside an area of 70×100 mm, as shown in FIG. 8A.

1. A multilayer printed wiring board in which insulating layers andconductor layers are stacked alternately on each other and the conductorlayers are electrically connected to each other through viaholes formedin the insulation layers, wherein the viaholes include a first group ofviaholes which are formed in one of two outermost insulating layers andanother insulating layer disposed inside the one insulating layer and asecond group of viaholes which are formed in the other of the twooutermost insulating layers and still another insulating layer disposedinside the other insulating layer; each of the viaholes included in thefirst and second groups is formed tapered to have a diameter which issmaller in the direction of thickness of the insulating layer; and theinsulating layer is about 100 μm or less in thickness.
 2. The multilayerprinted wiring board according to claim 1, wherein the insulating layeris about 50 μm or less in thickness.
 3. The multilayer printed wiringboard according to claim 1, wherein the first group of viaholes isstacked in such geometry that it is opposite to the second group ofviaholes.
 4. The multilayer printed wiring board according to claim 1,wherein the first group of viaholes is stacked in such geometry that itis shifted in relation to the second group of viaholes in a directiongenerally orthogonal to the direction of thickness of the insulatinglayers.
 5. The multilayer printed wiring board according to claim 1,wherein the viaholes in the first or second group are stacked on analmost straight line.
 6. The multilayer printed wiring board accordingto claim 1, wherein the viaholes in the first or second group arestacked together in such geometry that they are shifted from each otherin a direction generally orthogonal to the direction of thickness of theinsulating layer.
 7. The multilayer printed wiring board according toclaim 4, wherein the viaholes included in either the first or secondgroup are positioned at two opposite apexes of an imaginary tetragonallattice on the insulating substrate while the viaholes included in theother group may be positioned at other two opposite apexes of theimaginary tetragonal lattice on the insulating layer.
 8. The multilayerprinted wiring board according to claim 4, wherein the viaholes includedin either the first or second group are positioned at apexes of animaginary tetragonal or triangular lattice on the insulating substratewhile the viaholes included in the other group may be positioned at thecenter of the imaginary tetragonal or triangular lattice on theinsulating layer.
 9. The multilayer printed wiring board according toclaim 4, wherein the viaholes included in either the first or secondgroup are positioned at apexes of an imaginary triangular lattice on theinsulating substrate while the viaholes included in the other group maybe positioned at the center of the imaginary triangular lattice on theinsulating layer.
 10. The multilayer printed wiring board according toclaim 4, wherein the viaholes included in either the first or secondgroup are positioned intensively in a predetermined area of theinsulating substrate while the viaholes included in the other group maybe positioned in a peripheral area surrounding the predetermined area ofthe insulating layer.
 11. The multilayer printed wiring board accordingto claim 1, wherein each of the viaholes is tapered at an inner angle ofabout 60 to 90 degrees. toward the surface of the insulating substratein which the viahole is formed or the surface of the conductor circuitformed on the surface of the insulating layer.
 12. The multilayerprinted wiring board according to claim 1, wherein each of the viaholesis formed by filling a plating material in a hole formed in theinsulating substrate.
 13. The multilayer printed wiring board accordingto claim 1, wherein the viaholes included in the first and second groupsform a multistage stacked vias structure.
 14. A multilayer printedwiring board in which-one insulating substrate having a conductorcircuit formed thereon has stacked on either side thereof at least oneor more other insulating substrates having a conductor circuit formedthereon and the conductor circuit formed on the one insulating substrateand that on the other insulating substrate are electrically connected toeach other through viaholes formed in the insulating substrates,wherein: the viaholes formed in the other insulating substrate stackedon one side of the one insulating substrate form together a first groupincluding viaholes each formed tapered toward the surface of theinsulating substrate or that of the conductor circuit formed on thesurface of the insulating substrate, while the viaholes formed in theother insulating substrate stacked on the other side of the oneinsulating substrate form together a second group including viaholeseach formed tapered toward the surface of the insulating substrate orthat of the conductor circuit formed on the surface of the insulatingsubstrate; and the insulating substrate is about 100 μm or less inthickness.
 15. The multilayer printed wiring board according to claim14, wherein the insulating substrate is about 50 μm or less inthickness.
 16. The multilayer printed wiring board according to claim14, wherein the first group of viaholes is stacked in such geometry thatit is opposite to the second group of viaholes.
 17. The multilayerprinted wiring board according to claim 14, wherein the first group ofviaholes is stacked in such geometry that it is shifted in relation tothe second group of viaholes in a direction generally orthogonal to thedirection of thickness of the insulating substrates.
 18. The multilayerprinted wiring board according to claim 14, wherein the viaholes in thefirst or second group are stacked on an almost straight line.
 19. Themultilayer printed wiring board according to claim 14, wherein theviaholes in the first or second group are stacked together in suchgeometry that they are shifted from each other in a direction generallyorthogonal to the direction of thickness of the insulating substrate.20. The multilayer printed wiring board according to claim 17, whereinthe viaholes included in either the first or second group are positionedat two opposite apexes of an imaginary tetragonal lattice on theinsulating substrate while the viaholes included in the other group maybe positioned at other two opposite apexes of the imaginary tetragonallattice on the insulating substrate.
 21. The multilayer printed wiringboard according to claim 17, wherein the viaholes included in either thefirst or second group are positioned at apexes of an imaginarytetragonal or triangular lattice on the insulating substrate while theviaholes included in the other group may be positioned at the center ofthe imaginary tetragonal or triangular lattice on the insulatingsubstrate.
 22. The multilayer printed wiring board according to claim17, wherein the viaholes included in either the first or second groupare positioned at apexes of an imaginary triangular lattice on theinsulating substrate while the viaholes included in the other group maybe positioned at the center of the imaginary triangular lattice on theinsulating substrate.
 23. The multilayer printed wiring board accordingto claim 17, wherein the viaholes included in either the first or secondgroup are positioned intensively in a predetermined area of theinsulating substrate while the viaholes included in the other group maybe positioned in a peripheral area surrounding the predetermined area ofthe insulating substrate.
 24. The multilayer printed wiring boardaccording to claim 14, wherein each of the viaholes is tapered at aninner angle of about 60 to 90 degrees. toward the surface of theinsulating substrate in which the viahole is formed or the surface ofthe conductor circuit formed on the surface of the insulating substrate.25. The multilayer printed wiring board according to claim 14, whereineach of the viaholes is formed by filling a plating material in a holeformed in the insulating substrate.
 26. The multilayer printed wiringboard according to claim 14, wherein the viaholes included in the firstand second groups form a multistage stacked vias structure.
 27. Amultilayer printed wiring board in which an inner insulating substratehaving a conductor circuit formed thereon has stacked on either sidethereof at least one or more outer insulating substrates having aconductor circuit formed thereon and the conductor circuit formed on theinner insulating substrate and that on the outer insulating substrateare electrically connected to each other through viaholes formed in eachof the insulating substrates, wherein: the viaholes formed in the outerinsulating substrate stacked on one side of the inner insulatingsubstrate form together a first group including viaholes formed taperedtoward the surface of the insulating substrate or that of the conductorcircuit formed on the surface of the insulating substrate while theviaholes formed in the outer insulating substrate stacked on the otherside of the inner insulating substrate form together a second groupincluding viaholes formed tapered toward the surface of the insulatingsubstrate or that of the conductor circuit formed on the surface of theinsulating substrate; and the inner or outer insulating substrate isabout 100 μm or less in thickness.
 28. The multilayer printed wiringboard according to claim 27, wherein the insulating substrate is about50 μm or less in thickness.
 29. The multilayer printed wiring boardaccording to claim 27, wherein the first group of viaholes is stacked insuch geometry that it is opposite to the second group of viaholes. 30.The multilayer printed wiring board according to claim 27, wherein thefirst group of viaholes is stacked in such geometry that it is shiftedin relation to the second group of viaholes in a direction generallyorthogonal to the direction of thickness of the insulating substrates.31. The multilayer printed wiring board according to claim 27, whereinthe viaholes in the first or second group are stacked on an almoststraight line.
 32. The multilayer printed wiring board according toclaim 27, wherein the viaholes in the first or second group are stackedtogether in such geometry that they are shifted from each other in adirection generally orthogonal to the direction of thickness of theinsulating substrate.
 33. The multilayer printed wiring board accordingto claim 30, wherein the viaholes included in either the first or secondgroup are positioned at two opposite apexes of an imaginary tetragonallattice on the insulating substrate while the viaholes included in theother group may be positioned at other two opposite apexes of theimaginary tetragonal lattice on the insulating substrate.
 34. Themultilayer printed wiring board according to claim 30, wherein theviaholes included in either the first or second group are positioned atapexes of an imaginary tetragonal or triangular lattice on theinsulating substrate while the viaholes included in the other group maybe positioned at the center of the imaginary tetragonal or triangularlattice on the insulating substrate.
 35. The multilayer printed wiringboard according to claim 30, wherein the viaholes included in either thefirst or second group are positioned at apexes of an imaginarytriangular lattice on the insulating substrate while the viaholesincluded in the other group may be positioned at the center of theimaginary triangular lattice on the insulating substrate.
 36. Themultilayer printed wiring board according to claim 30, wherein theviaholes included in either the first or second group are positionedintensively in a predetermined area of the insulating substrate whilethe viaholes included in the other group may be positioned in aperipheral area surrounding the predetermined area of the insulatingsubstrate.
 37. The multilayer printed wiring board according to claim27, wherein each of the viaholes is tapered at an inner angle of about60 to 90 degrees. toward the surface of the insulating substrate inwhich the viahole is formed or the surface of the conductor circuitformed on the surface of the insulating substrate.
 38. The multilayerprinted wiring board according to claim 27, wherein each of the viaholesis formed by filling a plating material in a hole formed in theinsulating substrate.
 39. The multilayer printed wiring board accordingto claim 27, wherein the viaholes included in the first and secondgroups form a multistage stacked vias structure.
 40. A multilayerprinted wiring board in which insulating layers and conductor layers arestacked alternately on each other and the conductor layers areelectrically connected to each other through viaholes formed in theinsulating layers, wherein: the insulating layers are at least three innumber and less than 100 μm in thickness; the viaholes include a firstgroup of viaholes and a second group of viaholes; the first groupincludes viaholes in more than two stages of stacked vias extending inthe direction of thickness of the insulating layer and inwardly of themultilayer printed wiring board; and the second group includes viaholesformed tapered to have a diameter which is smaller away from the firstviahole group in the direction of thickness of the insulating layer. 41.The multilayer printed wiring board according to claim 40, wherein theinsulating substrate is about 50 μm or less in thickness.